Seabed Logging methods can be used to directly detect hydrocarbon reservoirs beneath the sea floor. These methods comprise deploying an electromagnetic source near the sea floor and measuring the response using one or more receiver instruments located at intervals spaced across the sea floor. The receiver instruments may be in the form of long rigid arms carrying electrical sensors, extending from a central body, which facilitate the detection of low-level electrical signals in seawater. The sensors at the end of the arms are therefore placed near or at the sea floor.
According to one existing method, instruments are deployed as follows: the positively buoyant instrument and an attached concrete anchor are dropped from a survey vessel at a chosen location; the instrument sinks freely to the sea floor; the position of the instrument while sinking may be monitored by acoustic methods using one or more transponders; the instrument is positioned near to or on the sea floor in a desired location which is held steady by means of the concrete anchor. The instrument is then used to measure and store data during a survey while it is located on or near to the sea floor. After the measurements are complete, acoustic commands from the sea surface cause the instrument to be released from the anchor; the instrument then floats up to the sea surface for retrieval by a survey vessel and the data is extracted from the instrument.
More specifically, a number of different systems may be combined in use to position a Seabed Logging source and one or more receivers prior to conducting a survey. These include, but are not limited to, acoustic transponders (mainly used for receiver positioning), magnetic compass systems (mainly used for orientation in the horizontal plane), depth transducers and altimeters (mainly used for orientation in the vertical plane), tilt and pitch sensors (for spatial orientation of the receivers), and gyro systems (for spatial and horizontal orientation). However, each of these has advantages and disadvantages when seeking the accuracy of data required to process and interpret the detected data to provide a 3D map. Examples of problems typically include acoustic ambient noise, sound reflections, ray bending and the varying sound transmission properties in salt water mainly caused by the variations in properties such as salinity and sea water temperature versus depth. This can wrongly image a target, or transponder, to be observed with a false offset both in range and in direction. At extreme angles, the target or transponder may even not be detected as it falls within a shadow zone caused by this ray bending. The magnetic direction may also locally vary from area to area and this deviation may additionally change some degrees in magnetic storm conditions and can be complicated to detect and compensate for. The resulting measurements can therefore include errors which are too large for use in 3D solutions.
An example of a problem is that known acoustic or electrical replying systems for positioning introduce external noise on the measured signals if the source is located in the near proximity of the highly sensitive Seabed Logging sensors. Such noise may introduce errors in the positioning and relative orientation measurements which may not be entirely removed during subsequent processing of the measured data. These errors may be amplified in a 3D analysis.
Higher degrees of accuracy in sensor positioning are required for new acquisition techniques, for example, 3D acquisition, as well as being desirable for improving the accuracy of results obtained from other processing techniques. Electric and magnetic fields are 3D vector dimensional and hence it is necessary to understand and include a full understanding of the spatial orientation of the sensors. There is therefore a need to improve the performance of receiver instruments, in order to improve the accuracy and efficiency with which surveys may be carried out and the acquisition of data may be undertaken.